As specified hereinbelow, the buffeting (the English term “buffet” is widely recognized) taken into consideration in the present invention corresponds to vibrations of the airplane, particularly in the cockpit, which are caused by aerodynamic effects applied to the structure of the wings, due to flow separation.
It is known that, in general, for performance reasons, it is preferable for an airplane to fly, in cruising flight, at a maximum permissible altitude. A maximum permissible altitude such as this is determined for an airplane chiefly on the strength of two criteria, namely a minimum rate of climb which has a predetermined value, for example 300 feet per minute (approximately 5.48 km/h), and minimal maneuverability. As far as this minimal maneuverability is concerned, aeronautic regulations dictate that an airplane, flying at its ceiling, that is to say flying at the maximum permissible altitude (for its weight) must be capable of effecting a 1.3 g maneuver, g being the acceleration due to gravity, which means that the angle of attack needs to be relatively large and that relatively high demands are made on the wings in terms of lift. Under such conditions, this maneuver is not limited by the maximum lift of the wings, which will generally allow a maneuver at a G-factor far higher than 1.3 g, but is limited by airplane vibrational phenomena known as buffeting, namely vibrations which are generated by aerodynamic effects applied to the structure of the wings and due to flow separation. These vibrations may be so great that they prevent the pilot from reading the flight instruments or from commanding the required G-factor of 1.3 g. For these reasons, aeronautic regulations have defined a buffet onset limit (the English term “buffet onset” is widely recognized) which represents a maximum acceptable level of vibration (in the cockpit). As a result, the aforementioned minimal maneuver at 1.3 g has to be performed before this buffet onset limit is reached or, at the extreme, at the moment this limit is reached. The purpose of dynamically reducing buffeting according to the invention is, therefore, to act on this limit with a view to making it easier to effect the aforementioned minimal maneuver.
It is known that buffeting of the aforementioned type occurs when a high level of lift is generated on a particular section of a wing, as a result of flow separation that creates unstable aerodynamic forces on the wing, which forces cause the structure of the airplane to vibrate. In general, flow separation appears initially on the out-board part of each wing.
There are various customary solutions for delaying this flow separation and therefore the onset of buffeting.
A first commonplace solution is to reduce the level of lift in the region of the wing concerned (the buffeting-generating region), for example its out-board part. In this case, greater lift is generated on the in-board part of the wing and a lower amount of lift is generated on the out-board part of the wing. A change in lift such as this may in particular be obtained by altering the twist of the wing. However, this customary solution modifies the lift distribution of the wing, which is generally defined for maximum performance in cruising flight, thus degrading wing performance during the flight and particularly in cruising flight (generally at 1 g).
A second customary solution is to mount vortex generators on the wings, these corresponding to mechanical elements that act on the flow and are intended to stabilize the flow in the region of each wing concerned and thus limit the structural response of this wing and therefore the level of vibrations. Such vortex generators improve the quality of the flow at high angles of attack, but lead to drag under normal flight conditions, because of their presence on the wings, and this may degrade airplane performance.
As a result, the aforementioned commonplace solutions capable of limiting the onset of buffeting have a negative effect on airplane flight performance and are therefore not satisfactory.